In the treatment of patients suffering respiratory ailments, such as emphysema where the patient's lung capacity is severely restricted, it is common practice to provide the patient with a source of oxygen enriched gas. Typically, this source of oxygen enriched gas is provided from a pressurized oxygen cylinder which may be located remotely from the patient in a hospital and supplied through suitable tubing (central storage type) or may be an individual cylinder located at the patient's bedside. Since many of these ailments are chronic and require extended therapy, portable oxygen cylinders which the patient may use at home have been developed.
While the use of individual cylinders provides the necessary life-sustaining therapy for these patients, the cylinders themselves present several problems when used in the home. Specifically, since these cylinders contain enriched oxygen gas, they present a constant danger of fire and explosion during use. The individual cylinders have limited capacity, and therefore must be serviced and replaced routinely thereby increasing the cost of therapy. In addition, there may also be leakage problems which may undetectedly diminish the capacity of a cylinder so that the patient is left with inadequate therapy gas.
Atmostpheric air, which contains about 20% oxygen and 78% nitrogen, provides a vast and abundant source of oxygen. However, until recently technology for extrating oxygen economically for individual use has been lacking. With the development of thin permselective membranes, such as those of plastics, such as silicone rubber, polyphenylene ethers and the like, and associate systems technology, feasible separation of gases has been achieved.
The separation of gases in such membrane systems technology is based on the selective permeability of certain materials. The term "selective permeability" means that one gas in a mixture will permeate through a membrane faster than a second gas, but this is not to suggest that one gas passes through the membrane to the complete exclusion of all others. Rather, a difference in the flow rate of two molecular species through a permeable membrane results so that the gas mixture on one side of the membrane is depleted in concentration of the more permeable component and the gas on the opposite side of the membrane is enriched with the more permeable component.
The oxygen-enricher systems associated with the membranes can comprise (i) those adapted to receive and compress atmospheric air before feeding compressed such air to the membranes for separation; and (ii) those in which the atmospheric air is fed to the membranes at ambient pressures and the oxygen enriched air, the so-called "permeate output" is drawn off under a partial vacuum.
In either case, to provide the highest efficiency as well as the most compact design, the membranes in oxygen enrichers are usually mounted on frames, to form envelope-like cells, and a plurality of such cells are arranged into layer-like stacks or arrays. It is important in all such systems to carry out the process with membranes that are leak-free, because air passing through leaking systems is obviously not efficiently enriched in oxygen, causing at best a reduced degree of oxygen enrichment and, at the worst, aggravation of the patient's condition due to no enrichment of the prescribed oxygen at all.
Previously, membrane leakage was determined by using an oxygen sensor in the extract line, or a valve to set up a reference flow to which the enriched air flow was compared. Neither method was entirely satisfactory, the former because frequent standardization was required, and the latter because the technique did not readily compensate for variations in stack output due to fluctuations in pressure, temperature and increasing age of the membranes.
The present invention is based on the discovery that if the membrane array is divided into two groups, or stacks, one can be used as an internal reference and membrane leakage is readily determined by pressure changes. The major advantages of this discovery are that there is no longer any need for an oxygen sensor; and that there is automatic compensation for variations in pressure, temperature, increasing age of the membranes, and changes in the ratio of permeate flow to feed flow.